In vitamin A, for example, and in its alkanoyl esters, the five conjugated double-bonds all have the trans-configuration. Of all the isomers, the all-trans vitamin A or its alkanoyl esters possess by far the greatest biological activity and are accordingly used almost exclusively in human and animal nutrition. Present commercial vitamin A preparations are almost exclusively of the synthetic type. However, the known and used processes for the manufacture of vitamin A and compositions thereof do not yield the pure all-trans compounds, but rather yield mixtures of various isomers containing more or less large amounts of the all-trans isomer. Therefore, there is the problem of isomerising the various isomers into the all-trans compound. This problem is especially acute since there is no good procedure for isomerizing the 9-cis compound into the all-trans compound. The problem of isomerism which arises is to achieve the highest possible yields of all-trans compounds and, also, since total cis-trans conversion is not possible, to obtain mixtures from which the all-trans isomer can be isolated in the easiest manner possible. The method most widely used has been isomerisation with iodine in the presence of pyridine. This method suffers from the disadvantage that only the 11-cis and 13-cis as well as 11,13-di-cis isomers, but not 9-cis or 9,13-di-cis isomers, can be converted into all-trans compounds.
The problem to be solved accordingly consists in finding an isomerisation process whereby there can be obtained, from the pure isomers or from any isomer mixtures thereof, a mixture which has the highest possible content of all-trans compound or a mixture from which the all-trans isomer can be readily isolated in the purest possible form.